JONESTOWN, Guyana (CNN) -- Cyanide was being bought and shipped to the Rev. Jim Jones' jungle compound in South America for at least two years before 909 Americans died there at the command of their cult leader, CNN has learned.

Sources in Guyana said the Jonestown camp began obtaining shipments of cyanide -- about a quarter to a half-pound of the deadly poison each month -- as early as 1976, well before most of Jones' followers made the move there.

CNN's Soledad O'Brien tells the story of the last hours of Jonestown -- and the few who did survive out of desperation and daring -- as CNN Presents "Escape from Jonestown."

Jones led his followers to their death after his gunmen killed a visiting congressman, Rep. Leo Ryan, and four others, including an NBC News correspondent and his cameraman, on November 18, 1978.

Jones told the members of his Peoples Temple church that the Guyanese Army would invade their settlement after the murders. He demanded that parents kill their children first, then take their own lives, rather than face the authorities because of what Jones had done.

Of the 909 who died, 303 were children -- from teens to toddlers. Many were killed by Jones' loyalists, who used syringes to squirt cyanide down their throats. Jones stockpiled cyanide »

CNN was told Jones obtained a jeweler's license to buy cyanide. The chemical can be used to clean gold. But there was no jeweler's operation in Jonestown.

Escape from Jonestown

Thirty years ago, more than 900 died by murder and suicide. Only 33 survived. Soledad O'Brien reports on their untold stories.Thursday, 9 p.m. ET

Six months before Ryan arrived on a one-man investigative mission, the settlement's doctor wrote in a memo to Jones:

"Cyanide is one of the most rapidly acting poisons. ... I would like to give about two grams to a large pig to see how effective our batch is."

The purchases are "strong evidence that the Rev. Jim Jones had been plotting the death of his followers long before that fateful day," O'Brien reports.

Ryan, the only U.S. representative assassinated in office, was shot at a nearby airstrip as he tried to leave with 15 church members who told him Jones was holding people captive in the remote jungle encampment.

"That was literally a jungle prison," said Gerald Parks, whose wife, Patricia, was shot to death in the airport attack. How did he escape death? »

Four other members of his family survived, including two young daughters who were lost in the jungle for three days after running away from the airstrip to hide from the killers. Woman returns to scene for first time »

"It was a dictatorship," said Vernon Gosney, who was badly wounded in the airport shootings. "It was supposed to be socialism, but it really was fascism."

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Jones was a phony faith healer who moved his Indiana church to northern California in the mid-'60s in search of a safe place to survive the possibility of nuclear warfare. In the mid-'70s, when a magazine raised questions about church beatings and financial abuses, Jones moved his flock to Guyana, in South America, to the jungle settlement he called his "beautiful promised land."

"It was a slave camp run by a madman," said Leslie Wilson, a young mother then only 21, who began walking away from Jonestown early on the day that ended in the suicides and murder.

She and 10 others trudged almost 30 miles through the jungle to another town. Wilson carried her 3-year-old son on her back. "It was a freedom walk," she said. "It was a walk to freedom."

Tim Carter, a Jones aide, stayed in the camp almost to the end and saw his wife and his 1-year-old son die before he was sent away on an errand.

Authorities made him return two days later to help identify bodies. Carter saw Jones lying with a bullet hole in the side of his head.

"I remember thinking the son of a bitch didn't even die the way everybody else died," Carter said.